1. Field of the Invention
The present invention relates to a device for heating crucibles.
2. The Prior Art
Heating elements are known for heating various containers, for example, crucibles. FIG. 1a is a longitudinal sectional view and FIG. 1b is a cross-sectional view through a generic prior art heating element for crucibles. The heating element has the shape of a cylindrical hollow body 1 which as a rule is made of graphite. The heating element is used in the preparation of crucible-pulled, single crystals composed of semiconductor material. The cylindrical hollow body is incised at intervals by slits 2, which start alternately from the upper or lower rim of the hollow body and extend almost to the respective opposite rim. The slits divide the hollow body into individual interlinked segments, which are referred to as meanders 3. Each meander is partially separated by a slit into a left and a right meander half 3a. At least two supply leads 4 are connected to the lower rim of the heating element. An electric power source is connected to the supply leads.
The diameter of the heating element may decrease towards the lower rim of the hollow body, so that the heating element is configured to match the shape of the crucible to be heated. For the sake of clarity and simplicity, the figures show embodiments having a constant diameter. However, the inventive concept is equally applicable to heating elements of any shape.
Normally, the upper rim of the heating element extends above the upper rim of the crucible to ensure that the crucible contents are uniformly heated by the thermal radiation emitted by the heating element. During the process of growing single crystals composed of semiconductor material, especially of silicon, molten semi-conductor material from the crucible often reaches the surface of the heating element. The rim of the heating element which extends above the crucible rim is particularly affected by this, since under certain doping conditions, it is possible for molten material to splash from the crucible. Moreover, semiconductor material which leaves the crucible in the gaseous state recondenses on the rim of the heating element. These deposits may, as in the case of silicon, react with the graphite of the heating element to produce carbidic phases which cause stresses in the heating element because of differential thermal expansion coefficients. These stresses are frequently resolved when portions spall off from the heating element and drop into the crucible while crystals are growing. There they significantly interfere with the dislocation-free growth of the single crystal. In particularly unfavorable cases, it is even necessary to terminate crystal growing because of such an incident. Repeating spalling of portions of the heating element greatly reduces its service life, requiring replacement at an earlier time. The disruption-free growing of crystals and the service life of the heating element are further effected by electrical flash-overs, which become more frequent as semiconductor material additionally condenses, in particular, in the vicinity of the lower rim of the heating element. This reduces the slit width between the meanders and/or the gap between the heating element and adjacent parts of the system.